Socket API

[FrankReh]

I noticed a TODO in the Socket's bind_internal method about possibly using or exposing the socket2::Socket's set_send_buffer_size and set_recv_buffer_size methods.

Should we have a builder api or should we expose the fd as a socket2::Socket or is it enough that as_raw_fd can be called on it?

I notice that the TcpListener and UnixListener do not provide an as_raw_fd.

If we know what we're doing now, I'd like to replace that TODO with a comment or documentation about how the user can set additional socket kernel parameters.

[Noah-Kennedy]

We should definitely provide AsRawFd, and also try and match the APIs from tokio (except for maybe TcpSocket)

[Icelk]

I definitely think the socket2 interface should be exported.

I'm thinking of using this for my multi-threaded web server. This being a single-threaded runtime only, I would set up multiple listeners with reuse_port enabled. That's however currently not possible (without transmuting, of course :)).

[Icelk]

This is fixed in master. Should we close?